The term “non-grain oriented magnetic steel sheet” is here taken to mean the magnetic steel sheets incorporated by DIN EN 10106 (“final annealed magnetic steel sheet”) and DIN EN 10165 (“non-final annealed magnetic steel sheet”). More strongly anisotropic types are also included as long as they are not grain-oriented magnetic steel sheets. To this extent the terms “magnetic steel sheet” and “magnetic steel strip” are used synonymously here.
“J2500” and “J5000” hereinafter designate the magnetic polarisation at a magnetic field strength of 2,500 A/m or 5000 A/m. “P 1.5” is taken to mean the magnetic reversal loss at a polarisation of 1.5 T and a frequency of 50 Hz.
The demand to provide non-grain oriented magnetic steel sheets of which the magnetic polarisation values are increased compared with conventional sheets is made by the processing industry. This applies in particular to the areas of application in which the electrical machines are electrically excited. Increasing the magnetic polarisation reduces the magnetisation requirement. This is associated with a decrease in copper losses which in a large number of electrical machines constitutes a fundamental portion of the losses occurring during operation of electrical machines.
The economical value of non-grain oriented magnetic steel sheets with increased permeability is considerable. Electrical machines with electrical excitation, specifically industrial drives with outputs which amount to 1 kW to 100 kW and above constitute the main area of application of non-grain oriented magnetic steel sheet.
The demand for higher permeable non-grain oriented types of magnetic steel sheet relates not only to non-grain oriented magnetic steel sheets with high losses (P1.5≧5-6 W/kg) but also sheets with medium (3.5 W/kg≦P1.5≦5.5 W/kg) and low losses (P1.5≦3.5). Therefore, the aim is to improve the entire spectrum of weak-, mid- and high-silicon electrotechnical steels with respect to its magnetic polarisation values. The types of magnetic steel sheet with Si contents of up to 2.5 weight % have particular importance with respect to their market potential.
Types of magnetic steel sheet which have a high magnetic polarisation value J2500 or J5000 with low magnetic reversal loss values P1.5 at 50 Hz, advantageously <4 w/kg, are specifically of interest as a reduction in the magnetic excitation current in the case of electrically excited machines and a reduction in the iron losses compared with conventional types of magnetic steel sheet with P1.5>4 W/kg at 50 Hz can take place.
A reduction in the magnetic reversal losses may be achieved by increasing the Si content. Considerably reduced losses are thus established if the total % Si+2% Al formed from the Si content and twice the Al content in steels used for producing magnetic steel sheets of the type in question is more than 1.4%.
Various methods are known as to how for magnetic steel sheets having such high contents of Si and Al high J2500 or J5000 can be achieved. For this purpose, it was thus proposed in EP 0 651 061 A1 to achieve high degrees of reshaping during cold rolling, wherein cold rolling can be carried out in two stages using intermediate annealing. It is also known that higher permeable types of magnetic steel sheets can be produced by intermediate annealing of the hot strip (EP 0 469 980 B1, DE 40 05 807 C2). In accordance with the method known from EP 0 431 502 A2 a non-grain oriented magnetic steel sheet is ultimately produced in that steel input stock containing ≦0.025% C, <0.1% Mn, 0.1 to 4.4% Si and 0.1 to 4.4% Al (amounts in weight %) is initially hot rolled to a thickness of not less than 3.5 mm. The hot strip thus obtained is then cold rolled, without recrystallising intermediate annealing, with a degree of deformation of at least 86% and is subject to an annealing treatment. The strip produced in accordance with the known method has a particularly high magnetic polarisation of more than 1.7 T at a field strength J2500 of 2500 A/m and low magnetic reversal losses.
However, in practice it has been found that using the known measures it is not possible, however, with the reliability required for large-scale production, to produce magnetic steel strips or sheets with total contents of Si and Al of more than 1.4 weight % which have a magnetic polarisation J2500 of ≧1.7 T, measured in the longitudinal direction of the strip. (The values ascertained for J2500 in the transverse direction of the strip and the multi-values of J2500 are always smaller than the values of J2500 measured in the direction of the strip).
Improvements with respect to higher values of J2500 may be achieved when high-silicon alloys of very high purity, specifically with very low Si and Ti contents with simultaneously low C content, are used. However, this method requires additional expenditure in the steel production compared with the FeSi steels conventionally used in practice.